The present invention relates to a method for handling calls to a non-registered terminal in a mobile telephone system, and then particularly in a system which operates without specific roaming numbers.
A public mobile telephone system that is intended to cover a wide area requires the application of advanced procedures to enable a call to a roaming subscriber to be directed to that radio base station which is in contact with the mobile telephone of the subscriber at that moment in time. Since fixed telephone networks, ISDN-networks and possibly other public land mobile telephone networks may be involved in setting up a single call connection, it is important that each point in the system can be quickly provided with information relating to the whereabouts of the roaming subscriber. Consequently, comprehensive signalling is required in order to keep the information in the system current and up to date.
Known mobile telephone systems are NMT (Nordic Mobile Telephone System), TACS (Total Access Mobile Telephone System) and AMPS (Advanced Mobile Phone Service), all of which are based on analog techniques, and GSM (Group Special Mobile), ADC (American Digital Cellular) and PDC (Personal Digital Cellular). NMT and TACS are described for instance in CMS88 Cellular Mobile Telephone System, ERICSSON document EN/LZT 101 908 R2, 1988, whereas the GSM-system is described in CME 20 System, ERICSSON document EN/LZT 120 226 R2A, 1991. ADC is described in the standardization document EIA/TIA IS 54, 55 and 56 and the network part in PDC is described in the standard Internode Specifications for Digital Mobile Communications Network, Ver. 3.2.
These mobile telephone systems use mobile services switching centres which are each connected to a number of radio base stations and which maintain contact with mobile subscribers roaming within the areas covered by said mobile services switching centres. These switching centres are connected to means, visitor registers, which keep an account of which subscribers are located within the service area of respective centres. The visitor location register may be common to a number of mobile services switching centres or each mobile services switching centre may be connected to an own visitor location register. A common visitor location register can cover the whole of a mobile telephone network.
Each subscriber is registered in a register in the network in which he/she is at home. The home location register contains subscriber related data, for instance mobile and subscriber authenticating data, and also data which discloses the location of the subscriber. For instance, if the subscriber roams into a network other than his home network, a reference is found to the visitor location register in this network, this reference having been transferred in a localization message in conjunction with the subscriber identifying himself when entering the new network. When making a call to a mobile station, each network is reached through a special gateway mobile services switching centre which with the aid of data in the address message of an external call collects information relating to the data record of the called subscriber from his home location register. The location of the subscriber in the network can then be traced with the aid of this information, therewith enabling a call connection to be set up. Paging and reporting are effected by means of signalling carried out with signalling protocol according to CCITT No. 7, Common Channel Signalling. The protocol MAP (Mobile Application Part) specified in CCITT recommendation Q.1051 is used between the gateway mobile services switching centre and the home location register and also between the home location register and the visitor location register, while the protocol ISUP (ISDN User Part) is used for signalling between the gateway mobile services switching centre and the mobile services switching centre and the visitor location register.
Normally, paging is effected in the known mobile telephone systems with the aid of a roaming number. A roaming number is, in principle, a telephone number which points to the data record of the subscriber in the visitor location register in which the subscriber is registered. In some systems, for instance the Japanese digital telephone system PDC whose network part has been standardized by I-Node Group (Internode Specifications--Digital Mobile Telecommunications Network, Ver. 3.2, 1992) a roaming number is also sent to the home location register immediately when a subscriber registers himself/herself. This roaming number is allocated by the network but does not belong to any specific subscriber, and the subscriber to which the roaming number has been allocated will normally retain the number while he roams in the service area of the visitor location register.
When a subscriber calls the mobile, by dialling the directory number of the subscriber, the call arrives at a gateway mobile services switching centre which handles incoming and outgoing signalling and traffic from the mobile network in which the called subscriber belongs. The number is analyzed and identifies the field in the home location register HLR in which all data concerning the subscriber is stored. The roaming number is read in the home location register and is then used to call the visitor location register within whose service area the subscriber is located. The roaming number is used to address the data record in the visitor location register in which the data of the called subscriber is registered. One drawback with the use of roaming numbers is that these numbers reduce the numbers in the numbering plan that are available as subscriber numbers. Another drawback is that a dynamic relationship exists between the roaming number and subscriber identity, because a roaming number points to different subscribers at different points in time, which makes it difficult to restore the information in the home and visitor location registers after an error or fault has occurred.
The Swedish Patent Application 9300721-9, which corresponds to commonly assigned U.S. patent application Ser. No. 08/205,154 filed Mar. 3, 1994, now U.S. Pat. No. 5,463,381, describes a method of identifying the location of a mobile subscriber which provides a direct connection with one of the numbers used in the system to clearly identify a subscriber, and which eliminates the aforesaid problems associated with roaming numbers. This is achieved by recording in the records of the subscriber in a home location register an identification number (PRN) which is unique for each visitor location register, and in the event of a call, using this number as the "CALLED ADDRESS" in an IAM-message in a ISUP-signal and by connecting to this message the subscriber directory number (MSN) and unique identification number (IMSI) which are used as a pointer to obtain access to the visitor location register GLR in which the subscriber is registered.